Feature: Dicke state

[vprusso]

Would be nice to add states/dicke_state.py.

For reference, QETLAB has a function here that implements this state. Also helpful, in Python, this implementation might be able to be cleaned up and incorporated into toqito (with providing proper credit to both sources in either case).

Creating the function in states/dicke_state.py would be part of the task, and the remaining part would be to add proper unit tests as well as a link in the states.rst file to the function.